Application of non-adhesive coatings, including acrylic melamine resin and the like, on bodies of automobiles has been increasingly employed recently. Regarding adhesion to such non-adhesive/non-adhesive coated boards, sufficient adhesion cannot be ensured even when a primer composition is used, and in particular, problems of poor initial adhesion and poor water resistant adhesion exist.
To increase adhesion, conventionally, carbon black has been blended to primer compositions. However, when carbon black is blended, problem exists in that discoloration due to the primer composition is conspicuous. Therefore, primer compositions that contain no carbon black and that can ensure adhesion to non-adhesive/non-adhesive coated boards have been demanded.
In response to such a demand, in Japanese Unexamined Patent Application Publication No. 2011-026546A, the inventors of the present technology have proposed “a primer composition containing: a polyester resin (A) having a number average molecular weight of 3000 or greater but less than 15000, and a polyisocyanate compound (B) having two or more isocyanate groups, where the polyester resin (A) has two or more aromatic carboxy groups”.
By further investigating the primer composition described in Japanese Unexamined Patent Application Publication No. 2011-026546A, the inventors of the present technology have found out that external appearance may be inferior because the primer composition may be discolored due to effect of ultraviolet ray or the like, depending on the type of polyisocyanate compound.